The present invention relates to digital cameras and other devices that utilize digital image sensors. More particularly, the present invention relates to methods of compensating for defective pixel elements in the image sensor of a device, and to devices implementing the methods.
Digital cameras are an example of devices that use image sensors. Digital cameras use a lens to focus light on a charge-coupled device (CCD) or other types of image sensors. The CCD includes an array of transistors that create electrical currents in proportion to the intensity of the light that strikes them. The transistors constitute the pixels of the image. A single pixel can include only one transistor for black-and-white photography, or can include several transistors for color photography. Generally, as used herein, the term pixel refers to individual sensor elements that can detect brightness (intensity), and optionally color (frequency). The higher the pixel density, the higher the resolution of the resulting image or photograph.
In response to light striking the CCD image sensor, the transistors generate analog electrical signals. An analog-to-digital (A/D) converter converts the analog electrical signals into a digital data stream. A digital signal processor, which functions as an imaging pipeline, adjusts the contrast and detail in the image, compresses the data that makes up the image so that it takes up less storage space, and sends the data to the camera's storage medium.
The image sensor is among the most expensive parts in a digital camera and other types of digital devices. Since a few bad pixels on the image sensor can ruin the quality of a picture, image sensors must have few or no defects. Image sensor defects typically take one of several forms. For example, entire rows or columns of pixels in the image sensor can be defective, being either always on (a dropin) or always off (a dropout). Typically, dropouts and dropins are stuck at full off (black) or full on (white) settings, which are very unusual in real-life photographs. In a photograph taken with a digital camera having a defective row or column of pixels, this defect would appear as a horizontal or vertical stripe.
Another type of pixel defect is bad spots, which are a region of pixels that are defective with dropouts and/or dropins. Also, in addition to these multiple pixel types of defects, individual pixels can be defective. Depending upon the required quality of the image sensor, and upon the location of a defective pixel, a single defective pixel can render an image sensor unusable.
Due to the above-described defects, manufacturing of image sensors frequency results in very low yields. These low yields increase the cost of image sensors, and thus increase the cost of digital cameras and other devices that utilize image sensors.